Re: 164 heater
I had the same problem with my 164 2.0l TS. Mine did not have aircon.
You are correct in your assumption.
There are two flaps. One for air mix, and one for air direction.
The plastic at the end of one of the flaps will have cracked, and the motor is now unable to turn it.
Getting to the air mixer box is really easy. It's done from the engine bay. First you remove the "leaf catcher" - or air intake grille - and then you can access the mixer box.
It's been a couple of years since I repaired mine, so I cant recall fully what needs to be disconnected to get the whole box out - but its not much - and I was suprised how easy it was.
Repairing the plastic spigot required a lot of Araldite and patience. It has to be totally solid and strong before re-assembly, or you'll get a repeat occurrence.
After Aralditing mine, it's been perfect for (as I mentioned) 2 years now.
I wish I had taken some photos of the process, but I totally forgot.
Alfa GT 1.9 JTD